China buys 80% of Iran's shipped oil. But this affects the prices of pretty much anyone who imports oil - if China can't get oil from Iran, they'll purchase it from someone else. Less supply => higher prices for everyone.
It also affects China less, they have huge strategic reserve, and massive coal to petchem industry that only becomes economic after $80 barrel. AKA all other big industrial players are energy exporters who has pay spot price. High oil price = PRC competitive advantage on top of being able to brrrt EVs / batteries to displace oil.
Prices are about the future, not the present. Even if this didn't immediately effect anything, it could signal that other energy infrastructure is a potential target, or merely that the war is going to last longer.
China has been buying Iranian crude and gas for many years, mostly due to simply needing as much as they can get.
The eleven tankers that have so far transited the Strait of Hormuz after the 3rd without being fired upon are all owned by, operated by and delivering to Chinese state-owned companies (AFAIK). The Iranians know they are transiting and are deliberately refraining from shooting, because of the Chinese information support, such as the Mizar-run near-realtime satellite geolocation of parked US aircraft and radars for attack.
This bit I'm adding is speculation but the likely role of the Marine detachment aboard the USS Tripoli which is currently being sent to the Gulf of Oman is to start sinking these China-bound tankers so as to use the offer of not sinking them to coerce China into supporting the reopening of rhe Strait.